1. Field of the Invention
This application relates generally to analog-to-digital conversion. More specifically, this application relates to a system and method for calculating an output from a sigma-delta analog-to-digital converter that minimizes power consumption.
2. Description of Related Art
Image sensing devices typically consist of an image sensor, generally an array of pixel circuits, as well as signal processing circuitry and any associated control or timing circuitry. Within the image sensor itself, charge is collected in a photoelectric conversion device of the pixel circuit as a result of the impingement of light.
One example of a pixel circuit is illustrated in FIG. 1. As shown in FIG. 1, a pixel circuit 100 includes a photoelectric conversion device 101 (e.g., a photodiode), a floating diffusion FD, a transfer transistor 102, a reset transistor 103, a source follower transistor 104, and a selection transistor 105, and a vertical signal line 106. As illustrated, vertical signal line 106 is common to a plurality of pixel circuits within the same column. Alternatively, a vertical signal line may be shared among multiple columns. Gate electrodes of transfer transistor 102, reset transistor 103, and selection transistor 105 receive signals TRG, RST, and SEL, respectively. These signals may, for example, be provided by the control or timing circuitry. Light falling on photoelectric conversion device 101 is converted into an analog electrical signal.
While FIG. 1 illustrates a pixel circuit having four transistors in a particular configuration, the current disclosure is not so limited and may apply to a pixel circuit having fewer or more transistors as well as other elements, such as capacitors, resistors, and the like. Additionally, the current disclosure may be extended to configurations where one or more transistors are shared among multiple photoelectric conversion devices.
The analog electrical signal in photoelectric conversion device 101 is retrieved by a readout circuit and is then converted to a digital value. Such a conversion typically requires several circuit components such as sample-and-hold (S/H) circuits, analog-to-digital converters (ADC), and timing and control circuits, with each circuit component serving a purpose in the conversion. For example, the purpose of the S/H circuit may be to sample the analog signals from different time phases of the photodiode operation, after which the analog signals may be converted to digital form by the ADC.
In an image sensor, there are typically a very large number of individual photoelectric conversion devices (e.g. tens of millions), and many ADCs working in parallel. Each ADC in the image sensor is shared by a large number of photoelectric conversion devices; for example, a column or multiple columns of photoelectric conversion devices may share a single ADC, a rectangular block of photoelectric conversion devices may share an ADC, and the like. As the number of pixels in the image sensor increases, the number of ADCs also increases. As a result, the power consumption of the ADCS becomes an increasingly large factor as the image resolution (i.e., number of pixels) increases. As the total power consumption in an image sensor in a mobile device increases, not only is there an increase in the power drawn from a battery and resultant decrease in operating time of the mobile device, but there is also an increase in the temperature of the silicon and resultant degradation in the noise performance of the sensor. Accordingly, there exists a need to improve the power consumption characteristics of an image sensor that uses sigma-delta ADCs.